In my previous post about Roman cooking I described a type of oven used to bake and roast food about 2,000 years ago. Known as a clibanus it was a sophisticated piece of cooking technology most likely used by the wealthy, and one with which I have spent many years experimenting.

These ovens were made with very course gritted clay and ranged in size from 15-50 cm in diameter, with walls of up to 10 cm high. A central hole seems to have been for regulating the temperature and could also allow cooks to keep an eye on the food baking inside. A flange allowed the fire to be placed on the top of the oven.

A replica oven being used

The sites in Italy where these ovens have been identified tend to be rather elite villa complexes where one could imagine the baking of delicate cakes and also warm bread for dinner. It is not necessarily apparent that these ovens were used by the less well off as a means of cooking simpler fair, and it is often assumed that they took their bread to be baked at large bakery complexes in towns, while we do not know what the rural poor did about baking at home. There was an assumption that they didn’t eat bread but made puls wheat porridge in a cooking pot over a wood fire.

The poem attributed to Virgil called moretum suggests that a relatively lowly market gardener baked his bread sub testu: under one of these ovens, though the status of this man is quite difficult to determine. He is considered a peasant, but he sells his produce in town and comes home with a heavy purse on occasion. Identifying these ovens is also not easy as they often come to pieces after prolonged use and recognising the shards (broken pieces) as testa (ovens) is problematic.

There is quite a detailed description of this oven in use in an agricultural manual written by Cato the Elder, in 150 BC. The recipe is for a special sacrificial cake called rather unfortunately placenta: the reason for which is another story altogether. It is a complex layered construction with sweet pasta sheets and cheese and honey bound in an outer pastry shell which might best be described as a round cheese strudel. While being constructed, the cook is advised to ‘heat the hearth and the testum where you are to cook’. Then ‘make the hearth ready beforehand and place the placenta on it, cover with a heated testum and place hot coals on top and around it’ (i).

From original drawings and reconstructed ovens the instructions on how to use it seemed quite logical, although my initial experiments were rather haphazard. I used a wood fire at first and rapidly broke the first oven I owned – we find evidence of metal versions of these ovens in Greece which could be used with a wood fire but ceramic, even very coarse material as these were, could not tolerate an open flame for long. Subsequently I used charcoal with much greater success.

Now I bake so often it has become second nature. The oven needs to be raised above the fire in order to be heated, so a trivet is used. Leaving the vessel directly on the fire caused rapid heat differential which caused cracking and sometimes put the fire that was inside the dome out.

A replica oven being used

I left the oven to heat over a charcoal fire and found that the hole acted as a chimney to draw the fire quite well. After a time it seemed better to close the hole and keep the heat in. The hearth I used was a raised platform and its position was crucial as a strong draft also helped the fire to heat. After about an hour or sometimes longer when the heat from the surface caused our spit to sizzle – a past visit to a wood-fired bakery had already told me that when the roof of these ovens is white hot that is when the baker knows it is ready to use.

At this point the hearth needs to be prepared which meant cleaning away the fire so that the cake or bread could go directly onto a tile or ceramic hearth. The fire was brushed aside to make space as big as the oven, and my freshly proved loaf or a Roman cake called a libum was placed on this hot surface with a bay leaf beneath for flavour.

The oven is put back over the cake and then the fire is piled around the sides and on top. A good bake requires good quality ‘restaurant charcoal’ (heavier and therefore long lasting) to retain plenty of latent energy. It is then possible to place fresh charcoal close to the already alight coals so that a continuous fire can be maintained. The remaining embers and smaller pieces of charcoal are pushed around the sides of the oven evenly spaced so that no area is left unheated. Doing this I was able to reach 410F (210C) on a regular basis when baking bread, so could then bake and roast very efficiently.

I can only indicate the quality of the bake by offering the following:

The crust on my sour dough was beautifully thick and crisp even when cooled; meat on the legs of a small chicken roasted for 45 minutes in a dish rather than directly on the hearth, fell to pieces; a lamb shank cooked for one and a quarter hours was similarly tender, and belly pork fell apart and had super crackling after the same amount of time.

Over the years I have come to the conclusion that the relatively small space inside the oven is such that any potential moisture both from bread and also meat is retained around the food being cooked so that a steam/roast/bake process is going on. Bakers know of course that you need steam to create a good crust and now it has become common to find modern catering ovens with added steam.

All manner of complicated techniques are used to achieve the desired moist atmosphere yet 2,000 years ago the Romans had invented the technique already.

I am a scientist specialising in metalworking technology, particularly in relation to non-precious metals such as iron and copper-alloys. The scientific examination and analysis of the Chiseldon Iron-Age cauldrons is a key aspect of the investigative process as a whole and is crucial in supporting our understanding of them.

For the Chiseldon cauldrons I have been examining the microstructure of the metal under very high magnification in order to see its composition, deduce how it was worked and explore manufacturing techniques. Some of the questions I will be trying to answer include: ‘How were the cauldrons made?’, ‘Were different components from an individual vessel made in the same workshop?’, ‘Were the same parts, such as the iron handles for different vessels, made from the same metal stocks’ and, perhaps the most important question of all; ‘Were the cauldrons made especially for burial or collected together for a particular occasion?’

Taking a sample from one of the cauldrons

Finding appropriate samples to test can be extremely difficult as the metal, particularly the iron, is extremely corroded and very fragile. The sampling process is made additionally complicated by attempting to sample a potential area that is as discrete as possible to make sure that we do not endanger the structural integrity of the artefact but will yield the best results. This is not a decision that is taken lightly and sample positions are chosen in consultation with curators and conservators. In order to reveal the structure of the metal the samples are mounted in resin, their cross-section polished, and then examined using metallographic microscopy up to x1000 magnification and a scanning electron microscope equipped with energy dispersive X-ray spectrometry (SEM-EDX) that allows us to examine them up to 300,000 times its actual size.

We have been able to deduce that the iron handles from both the cauldrons studied so far were probably formed by repeatedly hammering an iron bar while it was rotated. Additionally, iron used for the same parts of different cauldrons showed differences in microstructure and slag (impurity) inclusions, and was therefore from different stocks of metal, suggesting that these cauldrons were probably collected together rather than being made at the same time specifically for burial.

A high magnification scanning electron microscope (SEM) image of a copper alloy sample from one of the cauldrons. Darker horizontal lines were caused by many cycles of working and heating

The copper-alloy is likely to have been subjected to many cycles of working and annealing (heating) to reduce the sheet metal to its final thickness (and shape). Significantly, there are differences in the content of sulphide within the copper alloy from one of the cauldrons, which suggest that the metal of the bowl and that of the band were probably refined to different levels or were from different sources.

Some of the results we have achieved so far are intriguing and much more revealing than expected given the condition of the material. Further analysis of the remaining cauldrons will not only provide further details of how the metal was processed and how the cauldrons were made but will help us build up a more complete picture of the deposit as a whole.

The Chiseldon cauldrons research project is supported by the Leverhulme Trust

As illustrator in the Department of Prehistory and Europe at the British Museum, I am always looking for the best way to help our curators interpret an object. My role is to produce artwork that shows the form, construction and nature of objects and any decoration on them so that they can be clearly understood. This will usually result in illustration for academic publications and exhibitions as a result of research work on the Museum collection, or excavations.

Traditionally I have produced pen and ink technical drawings but new opportunities are available through various computer-generated methods of recording, analysing and understanding information about objects and excavation. With the Chiseldon Cauldrons material, I am exploring some of the possibilities of these new technologies, and in this case using photogrammetry and laser scanning programs to produce 3D records of the archaeological remains of individual cauldron blocks as they are excavated by Alex and Jamie.

Three-dimensional scan results

We are using a laser scanning system to produce 3D computer models of the individual pieces from each cauldron. This is achieved by plotting a thin red laser line as it is slowly moved across the surface of an object. Having recorded each piece we are hoping to use the photogrammetry programme to virtually re-construct the blocks as excavated.

The early results we achieved proved somewhat variable, especially with the laser scanning, but we are now starting to produce some very good quality scans in terms of both modelling and colour accuracy.

The plan is to produce virtual models of each cauldron as excavated which will enable us to understand much better what they would have originally looked like, and how they were made. It is also hoped that an overall plan of the pit and its contents can be reproduced. We’ll also be able to produce artwork for both printed and online publication, and to generate virtual re-constructions for publication and display. As we create interesting images, we’ll also post some of them here on the blog so you can see what we’re finding.

The Chiseldon cauldrons research project is supported by the Leverhulme Trust

As work on the Chiseldon Iron Age cauldrons progresses we are constantly making discoveries. Possibly the most exciting feature we have found so far is a decorated handle.

The decorated handle and section of rim came from a cauldron that had broken into several pieces during burial due to the weight of the overlying soil. Although we had used X-radiography to examine the handle fragment in its soil block before we began conservation, it was difficult to make out the surface due to the dense soil and corroded condition of the metal. This meant that when I was removing the soil I had to progress extremely slowly. However, it made discovering the decoration below especially exciting.

X-radiograph of the handle before conservation

The decoration consists of three curved plates that have been riveted below the rim on either side of and directly beneath the handle. The additional plates were carefully made and are likely not only to have been decorative, but also served to strengthen the point where the handle is attached.

Decorated handle after conservation.

While the plates could represent abstract decoration they strongly resemble a cow’s head, with the side-plates representing ears, the central plate a muzzle and the handle taking the form of boldly curved horns. Stylised decoration inspired by the shape of animals was not uncommon in the Iron Age and its association with feasting in this context is particularly relevant. However, decoration on cauldrons is extremely rare and this is a significant and exciting discovery.

Three-dimensional image of the handle

To help with the interpretation Stephen Crummy, an illustrator from the Department of Prehistory and Europe at the British Museum, has been scanning the decorated handle with a laser to make a three-dimensional image which will show its shape far more accurately and aid in creating a virtual reconstruction of the vessel.

The Chiseldon cauldrons research project is supported by the Leverhulme Trust

Iron Age cauldrons are rare, so when an excavation in Basel, Switzerland uncovered two Iron Age cauldrons in 2010, collaborations between the British Museum and the Swiss team were inevitable. In the spring, archaeologist Sophie Hugelin and conservator Janet Hawley visited us in London to see our work on the Chiseldon cauldrons. In early December Jamie Hood and I made a return visit to Basel.

The excavation site in Basel, Switzerland

The Swiss cauldrons are of a similar date and construction to the Chiseldon cauldrons and in a ‘pit’ deposit with a number of other ceramic and metal vessels, possibly as a result of ritual activity. But here the similarity of the find ends. Basel Gasfabrik is a large urban excavation on the banks of the river Rhine at the site of an old gasworks currently undergoing redevelopment. Jamie and I visited the site and were amazed at the vast scale of the excavation compared to the rural setting and small rescue excavation of the Chiseldon cauldrons.

With complex archaeological deposits the ideal method of excavation is to carry out a large three or four metre block lift of the entire deposit enabling further excavation work to be carried out in a more controlled manner away from the site. The scale and equipment required made a large lift impossible at Chiseldon, but at Basel Gasfabrik such equipment was readily available on the building site.

British Museum and Swiss conservators examine the cauldrons found in Basel

Although we had seen photographs of the find, seeing the block and cauldrons in person was fascinating and made the similarities in cauldron type with ours more readily obvious and recognisable. It was really valuable to exchange ideas about the archaeology and the conservation of the cauldrons with Janet and Sophie and see the different methods and approaches used.

It is amazing to think that over 2,000 years ago Iron Age man had cultural links hundreds of miles away on the continent, and through the discovery of these two finds we are now establishing links of our own with colleagues in Switzerland.

The Chiseldon cauldrons research project is supported by the Leverhulme Trust

With the Iron Age Chiseldon cauldrons excavated and cleaned to expose the metal surface we are beginning to see interesting technological features and evidence of manufacture revealing them to be sophisticated and high status objects.

Tool marks on the surface from the original manufacture

Although they’re over 2,000 years old, different tool marks from shaping and thinning the copper alloy are preserved on the surface of the metal. These suggest the careful and deliberate use of specific tools for different jobs, indicating that the objects were made by a craft specialist skilled at working sheet metal.

Faint incised lines marking-out the position of rivets

Other features likely to relate to construction are the lines, faintly incised into the surface of the sheet copper-alloy and only visible in raking light. These appear to mark the overlap of plates making up the sections of the cauldron, and the regular distribution and position of rivets indicating that the cauldrons were carefully designed and made.

Examination is also showing that, while the 12 cauldrons are broadly similar in their design, there are variations in their size, shape and construction. We have already identified three different types of rim construction. These differences are extremely intriguing and suggest that the cauldrons were made by different makers and/or at different times.

Multiple repair patches on the cauldron base

Another intriguing feature we are encountering is a high number of patched repairs. Some repairs appear to have been applied at the time of construction and placed over fatigue cracks caused by raising the metal. Others quite clearly cover areas of damage caused during the useful lifetime of the cauldrons, indicating that they were used and repaired over a period of time and were already old and well-loved items at the time of their burial.

Preserved details like this mean that while the cauldrons are in relatively poor condition there are minute pieces of evidence that allow us to build up a wider picture of how the cauldrons were designed and made, and really bring the objects to life by allowing us to see the craftsman’s thought process and the practical application of their art.

The Chiseldon cauldrons research project is supported by the Leverhulme Trust

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I am the curator responsible for the European Iron Age collection at the British Museum, and will be working with Alexandra Baldwin and Jamie Hood on the Chiseldon Cauldrons project throughout the next year.

At the moment I am taking a back seat in the project, to support Alex and Jamie as far as I can in their conservation work. But once the conservation and scientific analysis is completed it is up to me to work out why so many cauldrons were placed together in a large pit alongside two cattle skulls sometime between 200-50 BC.

In the meantime I have begun to research cauldrons and other metal vessels.

Cauldrons are a very well-known type of Iron Age artefact but surprisingly little is known about them. We think they were used to boil meat and/or to serve alcoholic beverages such as beer or mead. They are substantial artefacts and quite rare so we think they were used for feasting.

Part of the problem is that many cauldrons were discovered in rivers or bogs during the nineteenth-century so we have very little evidence to work with other than the artefacts themselves. This is why the Chiseldon discovery is so exciting. Because the objects were well-excavated we have a detailed record of how they were deposited. We also have up to 13 vessels to compare and contrast.

The discovery has certainly sparked a lot of interest among fellow archaeologists and I have already given a number of public lectures to various universities and archaeological societies.

Late last year I gave a lecture at Leicester University and there was a fantastic turnout. Usually one of the students bakes a cake or biscuits; however, in honour of the cauldrons we were treated to a steaming vat of punch served in a miniature cauldron!

I am extremely excited by what Alex and Jamie have discovered so far. One of the major questions we have is whether the cauldrons were made especially for deposition.

I think we can already suggest that they weren’t. The cauldrons that have been excavated so far are very different and look to have been made by different people using different techniques. Some also show possible evidence of repair and past use.

This is giving us a fantastic insight into Iron Age technology and methods of artefact manufacture. It also opens up further questions.

If cauldrons are rare artefacts and the examples we have were not all made at the same time, can we suggest that different communities brought their own vessels to a large feast at Chiseldon?

If so what was the purpose of the gathering and why were the artefacts placed in a pit at the end of the feast? We may not ultimately be able to answer these questions but I can’t wait to see what further discoveries Alex and Jamie make so we can at least try.

The Chiseldon cauldrons research project is supported by the Leverhulme Trust

As is often the case, the painstaking process of excavating the cauldron I’m working on has been more complex and time consuming than we initially thought.

The majority of the cauldron was lifted from its findspot in a block of soil supported by plaster bandages. Hundreds of smaller fragments were also removed from around the object during excavation. The fragmentary state of this cauldron is partially due to the fact that it was buried upside down, and over time the weight and the pressure of the overlying soil crushed and distorted it.

I began by carefully cutting away thin strips of plaster to reveal the top of the block and clear soil from the metal. With heavy clay soil, which is more solid than the objects, this has to be done with great care using scalpels and leaf trowels to remove soil dampened with water and alcohol.

Working down in layers, the sheet metal is uncovered, and the true condition of the object revealed. It is highly fragmentary and even undisturbed areas are in pieces.

Fragments loose in the soil have to be rejoined back onto larger sections immediately otherwise their location will be lost. This is done using thin tabs of nylon gossamer, a thin random weave of synthetic fibres, adhered over the join.

Removing the plaster support and soil from around the object makes it very unstable and because of this you are unable to see the entire object at once; the metal is so thin and fragile that it is unable to support its own weight.

I have now laid out the fragments from the excavation on a large table and have been looking for joins between them, and also between the fragments and metal contained in the soil block. Although I have found a number of joins there have been disappointingly few.

Very quickly it became apparent that there was something strange about this cauldron – there seemed to be too much copper alloy – several folded layers in the block as well as large sheet fragments.

From knowledge of other cauldrons we can tell that they are made in sections riveted together; the iron rim supporting the handles, then below this, two sections of copper alloy bowl. As I began to reveal more it appeared that there were two bases on top of each other – was this two cauldrons one inside another? Or were we looking at areas of a separate cauldron either displaced during burial or placed into the pit in fragments?

As each layer is revealed the position of the fragments is carefully recorded. Stephen Crummy, one of the Museum’s illustrators, has been using 3D laser scanning and photogrametery to map the block.

The next stage will be to try and decipher and interpret the remains. Then, after supporting and stabilising them, we will remove sections of the metal, effectively disassembling the object from around the soil.

The Chiseldon cauldrons research project is supported by the Leverhulme Trust

When an email came round from Allison Marccucci at Wessex Archaeology calling for volunteers to go field walking at the Chiseldon cauldrons find spot my colleague Jamie Hood and I volunteered enthusiastically.

Jamie had never been to the site before and knew it only from photographs and my hazy recollections. It was important for him to put the cauldrons into the context of the surrounding landscape, and we would both represent the British Museum and tell everyone what had been happening since 2005.

Clothed in waterproofs, wellington boots and several jumpers we walked out across muddy fields to the find spot. Winter is not the nicest time to be out on an exposed ridge in the Wiltshire countryside, but field walking has to be done at this time of year – after ploughing and before the crop growth obscures the ground.

As we approached I could see a cluster of people standing over the find spot. The original find had produced a lot of interest in the local area and in total 10 people had volunteered to field walk including Peter Hyams, the finder; John Winterburn, who did an initial excavation; and members of the local history society.

A grid of five metre squares was set out over the field and we walked across each square in pairs picking up fragments of pottery, worked stone and metal. The finds were bagged and their location recorded by square.

Further study of the fragments by Wessex Archaeology and their spread throughout the field will give an indication of the periods of activity and also the extent of the archaeological area. When combined with geophysics results it should help to place the cauldrons in context.

By this time it was bitterly cold and the rain had started driving across the field horizontally. Taking what shelter we could by the field boundary we ate a hasty lunch. Although unpleasant, the rain did have the advantage of washing the ground surface and making the potsherds more visible, but with darkness descending and the weather worsening we called it a day.

Despite the freezing rain it was great to be out in the field and talking to other people about the find. The importance of local knowledge to archaeology is vital, and often landowners and users know details of the local landscape that it would take archaeologists a long time to accumulate.

We have to remember that, although the objects have passed over to us in the British Museum to conserve and investigate, their importance is not only academic. The turnout for fieldwalking in less than ideal weather showed how important the cauldrons are to the people involved, something that can be easily forgotten when working back in the lab.

The Chiseldon cauldrons research project is supported by the Leverhulme Trust

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The two Chiseldon cauldrons we chose to work on first were found next to each other in the pit and had corroded together.

My colleague Jamie Hood has been given the first cauldron to be removed from the ground during excavation to work on.

Although it appeared to be in one piece in the ground, it was heavily corroded with large cracks hidden by the mud and soil. It was impossible to lift whole, so was removed in four large chunks surrounded by soil. The fact that it is in pieces actually makes it better for Jamie to work on as it is easier to move, handle, and support, and also fits under a microscope.

When he first started work within a few minutes I heard: ‘WOW, look at this, a charred seed!’ from the other side of the room.

Of all the cauldrons, it is in the worst condition – a chunk lifted in plaster bandages and a lot of small pieces of corroded metal. But, it might also be the most interesting.

Some small fragments of the copper alloy already cleaned have decorative scalloped edges, or, apparently, as decorative as it gets for cauldrons in the late Iron Age.

As yet we don’t know much more about this cauldron and wont until I excavate it from its soil block. However, due to its highly fragmentary condition it will not be possible to physically reconstruct it.

Instead I will try and concentrate on a virtual, or at least an intellectual reconstruction, trying to gain as much information from the fragments as possible.

The most important areas are the rim, handles and decorative patches, and if we can relocate these and examine how they were constructed then this will tell us a great deal about the cauldron.

To make things more complicated some of Jamie’s cauldron was corroded to and lifted with mine. Trying to decide which fragments of 0.5mm metal belong to which cauldron will be very difficult and the whole process will need very careful excavation and detailed recording.

The Chiseldon cauldrons research project is supported by the Leverhulme Trust